Method for etching organic film, method for fabricating semiconductor device and pattern formation method

ABSTRACT

An organic film is etched by using plasma generated from an etching gas containing a first gas including a straight chain saturated hydrocarbon and a second gas including a nitrogen component.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method for etching an organicfilm, a method for fabricating a semiconductor device and a patternformation method.

[0002] For the purpose of increasing the operation speed and loweringthe consumption power of semiconductor devices, decrease of thedielectric constant of an interlayer insulating film included in amulti-level interconnect structure is recently regarded as significant.In particular, an organic film with a small dielectric constant can beeasily formed by spin coating and curing, and hence is regarded as avery promising interlayer insulating film of the next generation. A wellknown example of the organic film with a small dielectric constant is anorganic film including an aromatic polymer as a base.

[0003] In order to fabricate a device with a refined design rule of agate length of 0.18 μm or less, a fine line processing technique ofapproximately 0.25 μm or less is necessary, and the design rule isconsidered to be more and more refined in the future. An organic film isgenerally patterned by plasma etching, but a fine pattern of 0.25 μm orless is very difficult to form from an organic film.

[0004] Known examples of the plasma etching employed for an organic filmare a process using an etching gas including a N₂ gas and a H₂ gas asprincipal constituents (reported by M. Fukusawa, T. Hasegawa, S. Hiranoand S. Kadomura in “Proc. Symp. Dry Process”, p. 175 (1998)) and aprocess using an etching gas including a NH₃ gas as a principalconstituent (reported by M. Fukusawa, T. Tatsumi, T. Hasegawa, S.Hirano, K. Miyata and S. Kadomura in “Proc. Symp. Dry Process”, p. 221(1999)).

CONVENTIONAL EXAMPLE 1

[0005] One of conventional etching methods will now be described asConventional Example 1 referring to the result obtained by etching anorganic film with a magnetic neutral loop discharge (NLD) plasma etchingsystem manufactured by Ulvac Corporation (“SiO₂ Etching in magneticneutral loop discharge plasma”, W. Chen, M. Itoh, T. Hayashi and T.Uchida, J. Vac. Sci. Technol., A16 (1998) 1594).

[0006] In Conventional Example 1, an organic film is etched by using anetching gas including a N₂ gas and a H₂ gas as principal constituents.The present inventors have carried out the etching process ofConventional Example 1 under the following conditions:

[0007] Plasma etching system: NLD plasma etching system Volume flowratio per minute in standard condition of etching gas:

N₂:H₂=50 ml:50 ml

[0008] Antenna power: 1000 W (13.56 MHz)

[0009] Bias power: 200 W (2 MHz)

[0010] Pressure: 0.4 Pa

[0011] Substrate cooling temperature: 0° C.

[0012] Etching time: 180 seconds

[0013]FIGS. 13A through 13D are cross-sectional SEM photographs of holesformed under the aforementioned etching conditions in organic films, andthe holes of FIGS. 13A through 13D have diameters of 0.16 μm, 0.18 μm,0.24 μm and 0.40 μm, respectively. In FIGS. 13A through 13D, a referencenumeral 101 denotes a silicon substrate, a reference numeral 102 denotesan organic film to be etched, and a reference numeral 103 denotes a maskpattern of a silicon oxide film used as a mask in etching the organicfilm 102. The organic film 102 has a thickness of approximately 1.02 μm,and the mask pattern 103 has a thickness of approximately 240 nm.

[0014] Conventional Example 1 is described as a process using theetching gas including, as principal constituents, a N₂ gas and a H₂ gas,and another method for etching an organic film is proposed as a processusing an etching gas including, as principal constituents, an O₂ gas, aN₂ gas and a C₂H₄ gas (Genexh Rajagopalan, et al.; Abstra. The 1999Joint International Meeting of ECS, Hawaii, October, 702 (1999)).

CONVENTIONAL EXAMPLE 2

[0015] Now, a method for fabricating a semiconductor device according toConventional Example 2 will be described with reference to FIGS. 14A and14B.

[0016]FIGS. 14A and 14B show states where an organic film 105 formed ona semiconductor substrate 104 is subjected to plasma etching by using amask pattern 106 of, for example, a silicon oxide film formed on theorganic film 105. FIG. 14A shows a state in the middle of the plasmaetching and FIG. 14B shows a state after completing the plasma etching.In FIGS. 14A and 14B, a reference numeral 107 denotes a first recesshaving a small diameter and a reference numeral 108 denotes a secondrecess having a comparatively large diameter. Although not shown in thedrawings, a metal material film is formed over the mask pattern 106 soas to fill the first recess 107 and the second recess 108, and a portionof the metal material film formed on the mask pattern 106 is removed by,for example, chemical mechanical polishing (CMP), so as to form aconnection plug or a metal interconnect from the metal material film.

[0017] As is shown in FIG. 14A, the etching rate of the first recess 107having a small diameter is lower than the etching rate of the secondrecess 108 having a comparatively large diameter.

[0018] Also, as is shown in FIG. 14B, the etching time required forcompleting etching the first recess 107 is generally calculated on thebasis of the etching rate of the first recess 107, and over-etching ofseveral tens % is generally conducted in addition to the calculatedetching time so as to completely remove the organic film 105 remainingon the semiconductor substrate 104 within the recess.

CONVENTIONAL EXAMPLE 3

[0019] As methods of forming a mask pattern through dry development, atop surface imaging (TSI) process, a three-layer resist process and thelike are known.

[0020] In the top surface imaging process, a surface of an organic filmresulting from pattern exposure is subjected to silylation, so as toselectively form a silylated layer on an exposed or unexposed portion ofthe organic film. Then, the organic film is subjected to dry developmentusing the silylated layer as a mask, so as to form a mask pattern.

[0021] In the three-layer resist process, after an organic film and asilicon oxide film are successively formed on a semiconductor substrate,a thin resist pattern is formed on the silicon oxide film. Then, thesilicon oxide film is subjected to plasma etching by using the resistpattern as a mask, so as to form an oxide film pattern by transferringthe resist pattern onto the silicon oxide film. Next, the organic filmis subjected to dry development (plasma etching) by using the oxide filmpattern. Thus, a fine organic film pattern having a high aspect ratio isformed from the organic film.

[0022] Furthermore, an etch target film formed on the semiconductorsubstrate is etched by using a two-layer mask pattern consisting of theoxide film pattern and the organic film pattern. In this manner, a finepattern that cannot be resolved by a single layer resist can be formedin the etch target film.

[0023] The present inventors have carried out the three-layer resistprocess, as a mask pattern formation method for Conventional Example 3,by using an etching gas including an O₂ gas under the following etchingconditions:

[0024] Plasma etching system: NLD plasma etching system

[0025] Flow rate per minute in standard condition of etching gas: O₂=90ml

[0026] Antenna power: 1000 W (13.56 MHz)

[0027] Bias power: 400 W (2 MHz)

[0028] Pressure: 0.133 Pa

[0029] Substrate cooling temperature: 0° C.

[0030] Etching time: 4 minutes

[0031]FIGS. 16A and 16B are cross-sectional SEM photographs of holesformed in an organic film pattern by the pattern formation method forConventional Example 3, and the holes of FIGS. 16A and 16B havediameters of 0.18 μm and 0.4 μm, respectively. In FIGS. 16A and 16B, areference numeral 111 denotes a silicon substrate, a reference numeral110 denotes an organic film pattern formed from an organic film, and areference numeral 109 denotes an oxide film pattern formed from asilicon oxide film. A resist pattern present on the oxide film pattern109 is eliminated during the formation of the organic film pattern 110through the dry development, and hence, an etch target film deposited onthe silicon substrate 111 is etched by using the two-layer mask patternconsisting of the oxide film pattern 109 and the organic film pattern110.

PROBLEM OF CONVENTIONAL EXAMPLE 1

[0032]FIG. 15 is a diagram of the RIE lag characteristic of the methodfor etching an organic film of Conventional Example 1. The RIE lag is aphenomenon that the etching rate is lowered when the aspect ratio of arecess to be etched is increased, which means that the etching rate islowered as the processing dimension (the dimension of an opening to beformed) is smaller in etch target films having the same thickness.

[0033]FIG. 15 shows the relationship between the diameter of a hole andthe etching depth obtained when holes having a diameter of 0.18 μmthrough 0.4 μm are formed in an organic film by conducting etching for180 seconds by using an etching gas of a mixed gas including a N₂ gasand a H₂ gas in a ratio in the flow rate (ml) per minute in the standardcondition, namely, N₂:H₂, of 0:100, 30:70, 50:50, 70:30 or 100:0. As isunderstood from FIG. 15, the typical RIE lag characteristic is observedin any of the ratios.

[0034] When the RIE lag characteristic is increased, a process marginsuch as allowance in etching amount is reduced in forming a finepattern. Therefore, when holes with different diameters or interconnectgrooves with different widths are formed together, excessive orinsufficient etching is caused in the respective holes or interconnectgrooves, which causes variation in the etching amount in underlyingfilms. As a result, the reliability of the semiconductor device islowered.

[0035] Moreover, since large over-etching is required for compensatingthe insufficient etching, variation in the dimension caused intransferring a pattern is increased. As a result, it is very difficultto highly precisely form a fine pattern.

PROBLEM OF CONVENTIONAL EXAMPLE 2

[0036] As described above, in the conventional method for fabricating asemiconductor device, the over-etching of several tens % is generallyconducted in addition to the calculated etching time. Therefore, whenthe etch point reaches the semiconductor substrate 104 in the secondrecess 108 (having a comparatively large diameter) with the high etchingrate, the etch point has not reached the semiconductor substrate 104 inthe first recess 107 (having a small diameter) with the low etchingrate.

[0037] Furthermore, as described above, the etching time is determinedon the basis of the time required for completing etching the firstrecess 107.

[0038] Accordingly, excessive over-etching disadvantageously occurs inthe bottom of the second recess 108.

[0039] Moreover, in the case where the etching time is insufficient,although the second recess 108 is sufficiently etched, the first recess107 is disadvantageously insufficiently etched.

[0040] Accordingly, in the case where holes having different diametersor interconnect grooves having different widths are formed together, theexcessive or insufficient etching is caused, which causes variation inthe etching amount in underlying films. As a result, the reliability ofthe semiconductor device is lowered.

PROBLEM OF CONVENTIONAL EXAMPLE 3

[0041] In Conventional Example 3, the dry development is carried out onthe organic film through the plasma etching using the etching gasincluding an O₂ gas as a principal constituent. Therefore, as is shownin FIGS. 16A and 16B, the hole formed in the organic film pattern 110has a diameter larger than the diameter of an opening of the oxide filmpattern 109, and the hole formed in the organic film pattern 110 has abowing cross-section. When the etch target film is etched by using theorganic film pattern 110 with the hole having such a bowingcross-section, it is difficult to highly precisely conduct the etching.

[0042] Therefore, in a method proposed for suppressing the hole of theorganic film pattern 110 from having a bowing cross-section, the drydevelopment is carried out on the organic film with the actual substratetemperature kept at a temperature below the freezing point by settingthe substrate cooling temperature (refrigerant temperature) to 20° C.through 50° C. below zero.

[0043] In order to attain such a low temperature, however, excessivecost and a large-scaled system are required, and hence, there ariseproblems of increase of the system cost and decrease of the systemstability. Therefore, it is not preferable that the substrate coolingtemperature is set to 20° C. through 50° C. below zero.

[0044] As described so far, the problem that the hole formed in theorganic film pattern 110 has a diameter larger than the diameter of theopening of the oxide film pattern 109 and the problem that the holeformed in the organic film pattern 110 has a bowing cross-section havenot been solved yet.

[0045] Needless to say, the problems occurring in the three-layer resistprocess occur in the top surface imaging process.

SUMMARY OF THE INVENTION

[0046] In consideration of the aforementioned conventional S problems, afirst object of the invention is stably and uniformly etching an organicfilm by minimizing a RIE lag characteristic so as to avoid excessive orinsufficient etching even when holes with different diameters orinterconnect grooves with different widths are formed together.

[0047] A second object of the invention is, in fabricating asemiconductor device including holes with different diameters orinterconnect grooves with different widths, improving the reliability ofthe semiconductor device by avoiding excessive or insufficient etchingso as to suppress variation in the etching amount in underlying films.

[0048] A third object of the invention is, in forming an organic filmpattern through dry development, highly precisely forming a mask patternwith a large process margin by preventing an opening of the organic filmpattern from having a dimension larger than the dimension of an openingof a mask used for forming the organic film pattern and by forming anopening with a vertical cross-section or a cross-section tapered towardthe bottom (hereinafter referred to as a forward taper cross-section) inthe organic film pattern.

[0049] In order to achieve the first object, the method for etching anorganic film of this invention comprises a step of etching an organicfilm by using plasma generated from an etching gas containing a firstgas including a straight chain saturated hydrocarbon and a second gasincluding a nitrogen component.

[0050] In the present method for etching an organic film, an organicfilm is etched by using plasma generated from the mixed gas containingthe gas including a hydrocarbon and the gas including a nitrogencomponent. Therefore, a deposition film is formed on an etch targetsurface, and owing to the deposition film, an ion assisted reaction iscaused on the bottom of a recess substantially without depending uponthe aspect ratio. Accordingly, a constant etching rate can be obtainedwithout depending upon the aspect ratio, namely, the diameter of therecess.

[0051] In particular, since a straight chain saturated hydrocarbon isused as the hydrocarbon in the method for etching an organic film, arecess with a vertical or forward taper cross-section can be formed inthe organic film with a very small RIE lag characteristic.

[0052] Accordingly, even when a fine pattern is to be formed, a processmargin such as allowance in etching amount can be large, and even whenholes with different diameters or interconnect grooves with differentwidths are to be formed together, excessive or insufficient etching canbe avoided, so that underlying films can be substantially uniformlyetched.

[0053] In the method for etching an organic film, the etching gaspreferably further contains a gas including a compound including carbon,nitrogen and hydrogen.

[0054] When the gas including a compound including carbon, nitrogen andhydrogen is thus mixed with the mixed gas containing the gas includingthe straight chain saturated hydrocarbon and the gas including thenitrogen component, a recess to be formed can attain a forward tapercross-section with keeping the very small RIE lag characteristic.Furthermore, by adjusting the mixing ratios of the mixed gas containingthe gas including the straight chain saturated hydrocarbon and the gasincluding the nitrogen component and the gas including the compoundincluding carbon, nitrogen and hydrogen, the angle of the forward tapercross-section and the RIE lag characteristic can be controlled. Thecompound including carbon, nitrogen and hydrogen may be methylamine.

[0055] In the method for etching an organic film, the first gas ispreferably a methane gas and the second gas is preferably a nitrogengas.

[0056] In this manner, a recess with a vertical or forward tapercross-section can be definitely formed in the organic film with a verysmall RIE lag characteristic.

[0057] In the method for etching an organic film, the etching gaspreferably further contains a rare gas.

[0058] In this manner, a deposition formed on the inner walls of areaction chamber used for the etching can be reduced.

[0059] The method for fabricating a semiconductor device of thisinvention comprises the steps of forming an organic film on asemiconductor substrate; forming, on the organic film, a mask patternincluding an inorganic compound as a principal constituent; and forminga recess in the organic film by selectively etching the organic film byusing the mask pattern and by using plasma generated from an etching gascontaining a first gas including a straight chain saturated hydrocarbonand a second gas including a nitrogen component.

[0060] In the present method for fabricating a semiconductor device, anorganic film is etched by using plasma generated from the mixed gascontaining the gas including hydrocarbon and the gas including anitrogen component, namely, a semiconductor device is fabricated by thepresent method for etching an organic film. Therefore, a recess with avertical or forward taper cross-section can be formed in the organicfilm with a very small RIE lag characteristic.

[0061] Accordingly, even when a fine pattern is to be formed, a processmargin such as allowance in etching amount can be large, and even whenholes with different diameters or interconnect grooves with differentwidths are to be formed together, excessive or insufficient etching canbe avoided, so as to substantially uniformly etch underlying films. As aresult, the reliability of the semiconductor device can be improved.

[0062] In the method for fabricating a semiconductor device, the etchinggas preferably further contains a gas including a compound includingcarbon, nitrogen and hydrogen.

[0063] In this manner, a recess to be formed can attain a forward tapercross-section with keeping the very small RIE lag characteristic, andthe angle of the forward taper cross-section and the RIE lagcharacteristic can be controlled by adjusting the mixing ratio, in theetching gas, of the gas including the compound including carbon,nitrogen and hydrogen. The compound including carbon, nitrogen andhydrogen may be methylamine.

[0064] In the method for fabricating a semiconductor device, the recesspreferably includes a via hole and an interconnect groove formed abovethe via hole and is filled with a metal material film by a dualdamascene method.

[0065] In this manner, the recess including the via hole and theinterconnect groove formed above the via hole, the via hole inparticular, can attain a forward taper cross-section with keeping thevery small RIE lag characteristic. As a result, good electric connectioncan be attained between a connection plug and a lower metal interconnectdisposed below the connection plug formed by the dual damascene method.Thus, the electric characteristic of a multi-level interconnectstructure formed by the dual damascene method can be improved.

[0066] In the method for fabricating a semiconductor device, the firstgas is preferably a methane gas and the second gas is preferably anitrogen gas.

[0067] In this manner, a recess with a vertical or forward tapercross-section can be definitely formed in the organic film with a verysmall RIE lag characteristic.

[0068] In the method for fabricating a semiconductor device, the etchinggas preferably further contains a rare gas.

[0069] In this manner, a deposition formed on the inner walls of areaction chamber used for the etching can be reduced.

[0070] The pattern formation method for this invention comprises thesteps of forming an organic film on a substrate; forming, on the organicfilm, a mask layer including an inorganic component; and forming anorganic film pattern from the organic film by selectively etching theorganic film by using the mask layer and by using plasma generated froman etching gas containing a first gas including a straight chainsaturated hydrocarbon and a second gas including a nitrogen component.

[0071] In the present pattern formation method, the organic film patternis formed by conducting selective etching on the organic film by usingplasma generated from the etching gas containing the gas including astraight chain saturated hydrocarbon and the gas including a nitrogencomponent, namely, the organic film pattern is formed by the method foretching an organic film of this invention. Therefore, an opening formedin the organic film pattern can be prevented from having a largerdimension than an opening of the mask layer, and an opening with avertical or forward taper cross-section can be formed in the organicfilm pattern with a very small RIE lag characteristic. Accordingly, amask pattern can be highly precisely formed with a large process margin.

[0072] In the pattern formation method, the etching gas preferablyfurther contains a gas including a compound including carbon, nitrogenand hydrogen.

[0073] In this manner, with keeping the very small RIE lagcharacteristic, the opening of the organic film pattern can attain aforward taper cross-section, and the angle of the forward tapercross-section and the RIE lag characteristic can be controlled byadjusting the mixing ratio, in the etching gas, of the gas containingthe compound including carbon, nitrogen and hydrogen. The compoundincluding carbon, nitrogen and hydrogen may be methylamine.

[0074] In the pattern formation method, the first gas is preferably amethane gas and the second gas is preferably a nitrogen gas.

[0075] In this manner, a recess with a vertical or forward tapercross-section can be definitely formed in the organic film pattern witha very small RIE lag characteristic.

[0076] In the pattern formation method, the etching gas preferablyfurther contains a rare gas.

[0077] In this manner, a deposition formed on the inner walls of areaction chamber used for the etching can be reduced.

[0078] In the pattern formation method, the mask layer is preferably asilylated layer.

[0079] In this manner, an opening with a vertical or forward tapercross-section can be formed in the organic film pattern with a verysmall RIE lag characteristic by the top surface imaging process.

BRIEF DESCRIPTION OF THE DRAWINGS

[0080]FIG. 1 is a diagram of the RIE lag characteristic of a method foretching an organic film according to Embodiment 1 of the invention;

[0081]FIGS. 2A, 2B, 2C and 2D are cross-sectional SEM photographs ofholes formed in an organic film by the method for etching an organicfilm of Embodiment 1;

[0082]FIGS. 3A, 3B, 3C, 3D and 3E are cross-sectional SEM photographs ofholes formed by a method for forming an organic film pattern accordingto Embodiment 1 for showing the relationship between mixing ratios of aCH₄ gas and a N₂ gas in an etching gas and the cross-sectional shape ofa hole with a diameter of 0.24 μm;

[0083]FIG. 4A is a diagram for explaining the mechanism of aconventional method for etching an organic film and FIG. 4B is a diagramfor explaining the mechanism of the method for etching an organic filmof Embodiment 1;

[0084]FIGS. 5A and 5B are cross-sectional SEM photographs of holesformed by a method for etching an organic film according to Embodiment 4of the invention, wherein FIG. 5A shows a hole formed by singly using aCH₃NH₂ gas and FIG. 5B shows a hole formed by using a mixed gas of a CH₄gas and a N₂ gas;

[0085]FIGS. 6A and 6B show the RIE lag characteristics of the method foretching an organic film of Embodiment 4, wherein FIG. 6A shows the RIElag characteristic obtained by singly using a CH₃NH₂ gas and FIG. 6Bshows the RIE lag characteristic obtained by using a mixed gas of a CH₄gas and a N₂ gas;

[0086]FIGS. 7A, 7B and 7C are cross-sectional views of holes formed bythe method for etching an organic film of Embodiment 4, wherein

[0087]FIG. 7A shows a hole formed in setting a flow rate of a CH₃NH₂ gas<<a flow rate of a (CH₄+N₂) gas,

[0088]FIG. 7B shows a hole formed in setting the flow rate of the CH₃NH₂gas>> the flow rate of the (CH₄+N₂) gas and FIG. 7C shows a hole formedin setting the flow rate of the CH₃NH₂ gas=the flow rate of the (CH₄+N₂)gas;

[0089]FIG. 8 is a diagram of the RIE lag characteristic of the methodfor etching an organic film of Embodiment 4;

[0090]FIGS. 9A, 9B and 9C are cross-sectional views for showingprocedures in a method for fabricating a semiconductor device accordingto Embodiment 6 of the invention;

[0091]FIGS. 10A, 10B and 10C are cross-sectional views for showing otherprocedures in the method for fabricating a semiconductor device ofEmbodiment 6;

[0092]FIGS. 11A, 11B and 11C are cross-sectional views for showingprocedures in a pattern formation method according to Embodiment 7 ofthe invention;

[0093]FIGS. 12A and 12B are cross-sectional views for showing otherprocedures in the pattern formation method for Embodiment 7;

[0094]FIGS. 13A, 13B, 13C and 13D are cross-sectional SEM photographs ofholes formed in an organic film by a method for etching an organic filmof Conventional Example 1;

[0095]FIGS. 14A and 14B are cross-sectional views for showing proceduresin a method for fabricating a semiconductor device of ConventionalExample 2;

[0096]FIG. 15 is a diagram for showing the RIE lag characteristic of themethod for etching an organic film of Conventional Example 1; and

[0097]FIGS. 16A and 16B are cross-sectional SEM photographs of organicfilm patterns formed by a pattern formation method for ConventionalExample 3.

DETAILED DESCRIPTION OF THE INVENTION

[0098] Embodiment 1

[0099] A method for etching an organic film according to Embodiment 1 ofthe invention will now be described with reference to FIGS. 1, 2Athrough 2D, 3A through 3E, 5A and 5B.

[0100] In the method for etching an organic film of Embodiment 1, amixed gas including, as principal constituents, a CH₄ gas and a N₂ gasis used as the etching gas, so as to etch an organic film with plasmagenerated from the mixed gas. Exemplified etching conditions inEmbodiment 1 are:

[0101] Plasma etching system: NLD plasma etching system Type of etchinggas and flow rates per minute in standard condition:

CH₄:N₂=30 ml:70 ml

[0102] Antenna power: 1000 W (13.56 MHz)

[0103] Bias power: 200 W (2 MHz)

[0104] Pressure: 0.4 Pa

[0105] Substrate cooling temperature: 0° C.

[0106] Etching time: 180 seconds

[0107]FIG. 1 is a diagram for explaining the RIE lag characteristic ofthe method for etching an organic film of this embodiment, which isobtained by etching an organic film with a thickness of approximately1.2 μm by using a mask pattern of a silicon oxide film with a thicknessof approximately 240 nm. FIG. 1 also shows the RIE lag characteristicsobtained when the ratio in the flow rate (ml) per minute in the standardcondition between the CH₄ gas and the N₂ gas included in the etchinggas, namely, CH₄:H₂, is 50 ml:50 ml, 70 ml:30 ml or 0 ml:100 ml.

[0108] As is understood from FIG. 1, the RIE lag characteristic isabrupt in forming a hole with a diameter of 0.2 μm or less when CH₄:N₂is 0 ml:100 ml.

[0109] Also, when CH₄:N₂ is 70 ml:30 ml, the RIE lag characteristic isobserved and the etching rate is lowered as a whole because of a highconcentration of the CH₄ gas. The reason why the etching rate is loweredwhen the concentration of the CH₄ gas is high will be described belowwith reference to FIGS. 4A and 4B.

[0110] When CH₄:N₂ is 50 ml:50 ml, the so-called inverse RIE lag thatthe etching rate is increased as the diameter of a hole is smaller isobserved.

[0111] On the other hand, when CH₄:N₂ is 30 ml:70 ml, no RIE lag iscaused, substantially a constant etching rate is obtained in all theholes having a diameter of 0.16 μm through 0.4 μm, and the etching rateis improved. In general, it is understood that a reaction is the mostefficiently proceeded in an etch target surface when the etching rate ismaximum, namely, when CH₄:N₂ is 30 ml:70 ml.

[0112]FIGS. 2A through 2D are cross-sectional SEM photographs of holesformed in an organic film by the etching method for Embodiment 1, andthe holes of FIGS. 2A through 2D have diameters of 0.16 μm, 0.18 μm,0.24 μm and 0.40 μm, respectively. In FIGS. 2A through 2D, a referencenumeral 1 denotes a silicon substrate, a reference numeral 2 denotes anorganic film with a thickness of approximately 1.2 μm, and a referencenumeral 3 denotes a mask pattern of a silicon oxide film with athickness of approximately 240 nm. At the beginning of the etching, aresist pattern with a thickness of approximately 0.4 μm is present onthe mask pattern 3, and is eliminated during the etching of the organicfilm 2.

[0113] As is understood from FIGS. 2A through 2C, when a hole has adiameter of 0.16 μm, 0.18 μm or 0.24 μm, a vertical cross-section can beobtained, and as is understood from FIG. 2D, when a hole has a diameterof 0.40 μm, a forward taper cross-section can be obtained.

[0114]FIGS. 3A through 3E are cross-sectional SEM photographs of holesfor showing the relationship, obtained in the pattern formation methodfor Embodiment 1, between the mixing ratios of the CH₄ gas and the N₂gas included in the etching gas and the cross-sectional shape of a holewith a diameter of 0.24 μm. FIGS. 3A through 3E show the cross-sectionsof the holes formed when the ratios in the flow rate (ml) per minute inthe standard condition between the CH₄ gas and the N₂ gas in the etchinggas, namely, CH₄:N₂, are 0 ml:100 ml, 30 ml:70 ml, 50 ml:50 ml, 70 ml:30ml and 100 ml:0 ml, respectively. In any cases, the etching is carriedout under the aforementioned etching conditions. In FIGS. 3A through 3E,a reference numeral 1 denotes a silicon substrate, a reference numeral 2denotes an organic film, a reference numeral 3 denotes a mask pattern ofa silicon oxide film, a reference numeral 4 denotes a resist pattern andreference numerals 5 and 6 denote depositions.

[0115] As is understood from comparison of FIG. 3B with FIGS. 3A, 3C, 3Dand 3E, the resist pattern 4 is removed through the etching only whenCH₄:N₂ is 30 ml:70 ml.

[0116] As is understood from FIGS. 3C and 3D, when the etching gasincludes 50% or more of the CH₄ gas, the resist pattern 4 remains andthe etching rate is lowered.

[0117] As is understood from FIG. 3E, when 100% of the etching gas isthe CH₄ gas, the resist pattern 4 is not etched and the depositions 5and 6 are formed. Specifically, after a hole is formed by etching theorganic film 2 to some extent, the deposition 6 is deposited within thehole.

[0118] The phenomena shown in FIGS. 3A through 3E reveal the following:If the resist pattern 4 can remain after completing the etching of theorganic film 2, the etching conditions can be selected from a wide rangewhere the amount of the CH₄ gas to be added to the etching gas isapproximately 70% or less. Alternatively, when the resist pattern 4cannot remain after completing the etching of the organic film 2, theamount of the CH₄ gas to be added to the etching gas is set to a rangehaving approximately 30% as the center for not allowing the resistpattern 4 to remain.

[0119] The holes of FIGS. 3A through 3E are obtained when the etchingtime is 180 seconds. In actual fabrication of a semiconductor device,namely, when over-etching is carried out, the etching time is longerthan 180 seconds. Therefore, the range of the amount of the CH₄ gas tobe added for not allowing the resist pattern 4 to remain is furtherenlarged. Depending upon an actually employed etching process, theamount of the CH₄ gas to be added may be 0% (shown in FIG. 3A) or 50%(shown in FIG. 3C).

[0120]FIGS. 4A and 4B are diagrams for explaining an effect of theetching method for this embodiment, wherein FIG. 4A shows the etchingmechanism obtained in using a conventional etching gas of a mixed gasincluding N₂ and H₂ or a NH₃ gas, and FIG. 4B shows the etchingmechanism obtained in using the etching gas of Embodiment 1 of the mixedgas including CH₄ and N₂. In FIGS. 4A and 4B, a reference numeral 1denotes a silicon substrate, a reference numeral 2 denotes an organicfilm to be etched, a reference numeral 3 denotes a mask pattern of asilicon oxide film, a reference numeral 7 denotes a radical fluxisotropically reaching the organic film 2, and a reference numeral 8denotes deposition film formed on the wall and the bottom of a recessformed in the organic film 2 by the etching.

[0121] In general, in conducting anisotropic etching by using plasma,the etching is mainly realized by proceeding an ion assisted etchingreaction, and is minimally proceeded by chemical sputtering, physicalsputtering and a thermochemical reaction as compared with the ionassisted etching reaction. In the ion assisted reaction, when ions arereleased from the plasma to reach an etch target film, the ions areaccelerated by an electric field of a plasma sheath region formedbetween a plasma generation region and the etch target film so as tocollide with the etch target film, resulting in proceeding a surfacechemical reaction in the vicinity of collision portions by collisionenergy. The mechanism of the etching through the ion assisted reactionis roughly divided into the following two types:

[0122] (First Etching Mechanism)

[0123] In the first mechanism, reactive radicals participating in theetching reaction are physically or chemically adsorbed onto the etchtarget surface, and this mechanism is further classified into thefollowing three cases:

[0124] In the first case, ions collide with the vicinity of the portionwhere the radicals are adsorbed, so as to cause a chemical reactionamong the ions, the adsorbed substance and the material of the etchtarget film.

[0125] In the second case, the adsorption is further proceeded so as toform a thin deposition film on the etch target surface, and also in thiscase, the ion assisted reaction can be efficiently proceeded through theion collision, resulting in attaining a high etching rate.

[0126] In the third case, the deposition film has a large thickness, andin this case, most of ions reaching the etch target surface are consumedin removing the deposition film, and hence, the etching rate isexcessively lowered. Also, when the thickness of the deposition film islarger than a predetermined value, namely, too large to remove throughthe ion collision, the deposition film cannot be removed by the ions.Therefore, the chemical reaction among the ions reaching the etch targetsurface, the adsorbed substance and the material of the etch target filmis terminated, resulting in stopping the etching.

[0127] (Second Etching Mechanism)

[0128] In the second mechanism, no reactive radicals participating inthe etching reaction is adsorbed onto the etch target surface. In thiscase, ions collide with the etch target surface and cause a chemicalreaction with the material of the etch target film directly by theenergy of the ions themselves, so as to proceed the ion assisted etchingreaction.

[0129] In the case where the plasma generated from the mixed gasincluding N₂ and H₂, radicals (hereinafter, reactive neutral particleswith activity including atoms are generally designated as radicals)generated in the plasma are considered to be N, N₂, H and H₂, and in thecase where the plasma of a NH₃ gas is used, generated radicals areconsidered to be not only N, N₂, H and H₂ but also NH, NH₂ and NH₃.Therefore, when the NH₃ gas is used, the amount of generated radicals islarger as compared with the case where the mixed gas of N₂ and H₂ isused, and hence, the amount of substances adhered onto the etch targetsurface is probably increased. However, the adsorbed substance is notresistant to the collision of ions emitted from the plasma to the etchtarget surface, and hence, a deposition film is not formed on the etchtarget surface.

[0130] Accordingly, the etching reaction occurring on the etch targetsurface on the bottom of the recess is probably dominantly a reaction toetch a small amount of atoms or molecules adhered onto the etch targetsurface and atoms present on the surface of the organic film by the ionassisted reaction caused by the ions emitted from the plasma (by themechanism of the first case of the first etching mechanism), or anetching reaction between the ions and the etch target surface (by thesecond etching mechanism). In particular, in the etching by using theconventional plasma of N₂ and H₂, the etching is probably dominantlyproceeded by the second etching mechanism.

[0131] Since a hydrogen ion is small in its atomic radius and inertialmass, it probably enters the inside of the organic film without causinga reaction when it reaches the etch target surface. Therefore, it seemsthat nitrogen ions and ions of ammonia fragments (molecules and atomsgenerated through dissociation and decomposition from ammonia molecules)are the prime cause for proceeding the ion assisted reaction.

[0132] In general, an organic film includes, as a principal constituent,a polymer consisting of carbon atoms and hydrogen atoms, and the organicfilm is etched by nitrogen or hydrogen radicals and ions reaching theorganic film as in Conventional Example 1. Therefore, it seems that aprincipal reaction product generated in the etching is volatile HCN andthat the etching is proceeded by releasing the HCN from the etch targetsurface.

[0133] In general, the radical flux is lowered in the bottom of a recesshaving a higher aspect ratio. In contrast, ions comparatively constantlyreach the bottom of a recess substantially regardless of the aspectratio of the recess.

[0134] Accordingly, the etching rate is lowered in a recess having ahigher aspect ratio where the radical flux is lowered, namely, a recesshaving a smaller diameter. This is the prime cause of the RIE lag. To beextract, the RIE lag may also be caused by the ion flux lowereddepending upon the aspect ratio. on the contrary, when the plasmagenerated from the mixed gas including CH₄ and N₂ is used as inEmbodiment 1, radicals of N, N₂, C, H, CH, CH₂, CH₃ and CH₄ aregenerated. Specifically, Embodiment 1 is different from ConventionalExample 1 in a first point that there exist radicals of CH_(x) (whereinx is 1, 2, 3 or 4). These CH_(x) radicals tend to form a polymer on theetch target surface, and hence, the deposition film 8 is formed on theetch target surface, so as to fix atoms required for the etchingreaction onto the etch target surface. The deposition film 8 with anappropriate thickness has a function as a reaction layer, so as to serveas the prime cause for efficiently causing the ion assisted reaction (bythe mechanism of the second case of the first etching mechanism).

[0135] In Embodiment 1, the etching gas of the mixed gas including theCH₄ gas and the N₂ gas is used, the deposition film 8 is formed on theetch target surface, and the deposition film 8 fixes atoms required forthe etching reaction onto the etch target surface. Therefore, thedeposition film 8 has a function to compensate the phenomenon that theradical flux is lowered on the bottom of a recess with a higher aspectratio.

[0136] Also, as described above, the ions comparatively constantly reachthe bottom of a recess substantially regardless of the aspect ratio.

[0137] Accordingly, substantially without depending upon the aspectratio, the ion assisted reaction is caused on the bottom of the recess,so that the constant etching rate can be obtained regardless of theaspect ratio, namely, the diameter of the opening.

[0138] When the aforementioned phenomena are more strictly observed, theradical flux reaching the bottom of a recess is actually lowered as theaspect ratio of the recess is higher, and hence, the thickness of thedeposition film formed on the bottom of the recess is varied inaccordance with the proceeding of the etching. Accordingly, as is shownin FIG. 1, different RIE lag characteristics are observed depending uponthe mixing ratios of the CH₄ gas and the N₂ gas.

[0139] Now, the influence of the mixing ratios of the CH₄ gas and the N₂gas in the etching gas upon the RIE lag characteristic will be describedin more detail.

[0140] First, in the case where no CH₄ gas is added, namely, when theetching gas includes the N₂ gas alone, a conspicuous RIE lagcharacteristic is observed for the same reason as in ConventionalExample 1.

[0141] Next, in the case where the CH₄ gas is included in approximately30%, the RIE lag characteristic is not observed for the followingreason: Since the radical flux reaching the bottom of a recess islowered as the aspect ratio of the recess is higher, although thethickness of the deposition film 8 depends upon the diameter of therecess, the deposition film 8 in an amount equal to or sufficientlylarger than an amount consumed in the ion assisted reaction is formed onthe bottom of a recess having a small diameter. Therefore, asubstantially constant etching rate is obtained without depending uponthe diameter of the recess, namely, the aspect ratio. This factparadoxically reveals that the surface adsorption coefficient (surfaceadsorption probability) of the CH_(x) radicals is small and that thetransfer loss to the inside of a hole or interconnect groove with a highaspect ratio is considerably small.

[0142] Next, in the case where the CH₄ gas is included in approximately50%, the amount of radicals that can reach the etch target surface onthe bottom of a recess basically from the uppermost face of thesubstrate (namely, the surface of the mask pattern 3) depends upon theaspect ratio, and when the amount of radicals supplied onto theuppermost face of the substrate is increased, the amount of the radicalsreaching the bottom of the recess is naturally increased. At this point,part of ions reaching the bottom of the recess are consumed in removingan excessive deposition, and hence, the etching rate is lowered as awhole. Furthermore, as the aspect ratio is lower, namely, as thediameter of the opening is larger, the deposition film 8 is formed onthe bottom of the recess in an amount larger than the amount necessaryfor the ion assisted reaction. Accordingly, as the deposition film 8 hasa larger thickness, a larger part of the ions reaching the bottom of therecess is consumed in removing the excessive deposition film 8. As aresult, the amount of ions consumed in removing the deposition film 8 issmaller in a recess with a smaller diameter than in a recess with alarger diameter, and therefore, the so-called inverse RIE lag phenomenonthat the etching rate of a recess with a smaller diameter is larger thanthe etching rate of a recess with a larger diameter is observed.

[0143] Next, the case where the CH₄ gas is included in approximately 70%will be examined. As described with respect to the etching method forConventional Example 1 referring to FIG. 4A, ions of nitrogen atoms ornitrogen molecules are the prime cause for proceeding the ion assistedreaction. When the etching gas includes 70% of the CH₄ gas, the supplyamount of the N₂ gas is naturally reduced, and hence, the amount ofgenerated ions of nitrogen atoms or nitrogen molecules serving as theprime cause of the ion assisted reaction is reduced. Accordingly, thereduced amount of the ions of nitrogen atoms or nitrogen molecules alsolowers the etching rate when the amount of the CH₄ gas to be added isincreased.

[0144] Furthermore, when the CH₄ gas is included in approximately 70%,the thickness of the deposition film 8 is further increased, and hence,a larger amount of ions are consumed in removing the deposition film 8.In addition, since the supply amount of the N₂ gas indispensable to theetching reaction is reduced, the etching rate is further lowered as awhole. Simultaneously, as is understood from FIG. 3D, the resist pattern4 formed on the mask pattern 3 of the silicon oxide film is minimallyetched. Accordingly, when the etching is proceeded to some extent, theaspect ratio is excessively increased in a recess with a small diameter,so that the amounts of radicals and ions supplied to the bottom of therecess can be abruptly reduced. Therefore, the etching rate of therecess with a small diameter is excessively lowered. As a result, in thecase where the CH₄ gas is included in 70% in which the resist pattern 4is minimally etched, the RIE lag occurs again.

[0145] Embodiment 2

[0146] A method for etching, an organic film according to Embodiment 2of the invention will now be described.

[0147] In the method for etching an organic film of this embodiment, amixed gas including, as principal constituents, a CH₄ gas and a NH₃ gasis used as the etching gas, so as to etch an organic film with plasmagenerated from the mixed gas. Exemplified etching conditions inEmbodiment 2 are:

[0148] Plasma etching system: NLD plasma etching system Type of etchinggas and flow rates per minute in standard condition:

CH₄:NH₃=30 ml:70 ml

[0149] Antenna power: 1000 W (13.56 MHz)

[0150] Bias power: 200 W (2 MHz)

[0151] Pressure: 0.4 Pa

[0152] Substrate cooling temperature: 0° C.

[0153] An effect of the etching method for Embodiment 2 will now bedescribed.

[0154] Embodiment 2 is different from Embodiment 1 in using the NH₃ gasinstead of the N₂ gas, which results in attaining a higher etching ratein Embodiment 2 than in Embodiment 1.

[0155] The effect attained by adding the CH₄ gas to the etching gas isthe lowering of the RIE lag characteristic as described in Embodiment 1.

[0156] Since the mixed gas including the CH₄ gas and the NH₃ gas as theprincipal constituents is used for etching an organic film in Embodiment2, the improvement of the etching rate and the lowering the RIE lagcharacteristic can be both realized.

[0157] Embodiment 3

[0158] A method for etching an organic film according to Embodiment 3 ofthe invention will now be described.

[0159] In the method for etching an organic film of Embodiment 3, amixed gas including, as principal constituents, a CH₄ gas, a N₂ gas anda H₂ gas is used as the etching gas, so as to etch an organic film withplasma generated from the mixed gas. Exemplified etching conditions inEmbodiment 3 are:

[0160] Plasma etching system: NLD plasma etching system Type of etchinggas and flow rates per minute in standard condition:

CH₄:N₂:H₂=30 ml:35 ml:35 ml

[0161] Antenna power: 1000 W (13.56 MHz)

[0162] Bias power: 200 W (2 MHz)

[0163] Pressure: 0.4 Pa

[0164] Substrate cooling temperature: 0° C.

[0165] An effect of the etching method for this embodiment will now bedescribed.

[0166] Since the mixed gas including the CH₄ gas, the N₂ gas and the H₂gas as principal constituents is used in Embodiment 3, not only a higheretching rate than in Embodiment 1 can be obtained but also the amountsof N₂ and H₂ included in the etching gas can be more preciselycontrolled than in Embodiment 2. Accordingly, the etching can be carriedout under more preferable conditions than in Embodiments 1 and 2.Specifically, Embodiment 3 can realize a process with a large processwindow.

[0167] The effect attained by adding the CH₄ gas to the etching gas isthe lowering of the RIE lag characteristic as described in Embodiment 1.

[0168] Since the mixed gas including the CH₄ gas, the N₂ gas and the H₂gas as the principal constituents is used for etching an organic film inEmbodiment 3, not only the improvement of the etching rate and thelowering of the RIE lag characteristic can be both realized but also thecontrollability of the etching and the process window can be increased.

[0169] Embodiment 4

[0170] A method for etching an organic film according to Embodiment 4 ofthe invention will now be described with reference to FIGS. 5A, 5B, 6A,6B, 7A through 7C and 8.

[0171] In the method for etching an organic film of Embodiment 4, amixed gas including, as principal constituents, a CH₄ gas, a N₂ gas anda CH₃NH₂ (methylamine) gas is used as the etching gas, so as to etch anorganic film with plasma generated from the mixed gas. Exemplifiedetching conditions in Embodiment 4 are:

[0172] Plasma etching system: NLD plasma etching system

[0173] Type of etching gas and flow rates per minute in standardcondition:

CH₄:CH₃NH₂:N₂=15 ml:50 ml:35 ml

[0174] Antenna power: 1000 W (13.56 MHz)

[0175] Bias power: 200 W (2 MHz)

[0176] Pressure: 0.4 Pa

[0177] Substrate cooling temperature: 0° C.

[0178] Now, an effect of the method for etching an organic film ofEmbodiment 4 will be described in contradiction to etching of an organicfilm by using an etching gas including a CH₃NH₂ gas as a principalconstituent (in a flow rate per minute in the standard condition of theCH₃NH₂ gas of 100 ml) and etching of an organic film by using an etchinggas including a CH₄ gas and a N₂ gas as principal constituents (in aratio in the flow rate per minute in the standard condition between CH₄and N₂ of 30 ml:70 ml). The plasma etching system, the antenna power,the bias power, the pressure and the substrate cooling temperatures arethe same in all the etching as those employed in Embodiment 4.

[0179]FIG. 5A is a cross-sectional SEM photograph of a hole formed bysingly using the CH₃NH₂ gas, and FIG. 5B is a cross-sectional SEMphotograph of a hole formed by using the mixed gas of CH₄ and N₂. InFIGS. 5A and 5B, a reference numeral 1 denotes a silicon substrate, areference numeral 2 denotes an organic film, a reference numeral 3denotes a mask pattern of a silicon oxide film, and the bottom of anopening formed in the mask pattern 3 has a diameter of 0.24 μm. A resistpattern with a thickness of approximately 0.4 μm formed on the maskpattern 3 is eliminated during the etching of the organic film 2.

[0180]FIG. 6A shows the RIE lag characteristic obtained by singly usingthe CH₃NH₂ gas and FIG. 6B shows the RIE lag characteristic obtained byusing the mixed gas of CH₄ and N₂.

[0181] When the CH₃NH₂ gas is singly used, the hole has a good forwardtaper cross-section as is understood from FIG. 5A but the RIE lagcharacteristic is not very good as is understood from FIG. 6A.

[0182] On the other hand, when the mixed gas of CH₄ and N₂ is used, thehole has a substantially vertical cross-section as is understood fromFIG. 5B and the RIE lag characteristic is good as is understood fromFIG. 6B. In other words, a constant etching rate is obtained withoutdepending upon the diameter of the hole.

[0183] In Embodiment 4, the ratio between the constituents of theetching gas (CH₄:CH₃NH₂:N₂=15 ml:50 ml:35 ml) is obtained by mixing ahalf of the flow rate per minute of the singly used CH₃NH₂ gas (namely,100 ml) and a half of the flow rates per minute of the mixed gas of CH₄and N₂ (namely, CH₄: NH₂=30 ml:70 ml). Accordingly, Embodiment 4 canattain a characteristic intermediate between the characteristic obtainedby singly using the CH₃NH₂ gas and the characteristic obtained by usingthe mixed gas of CH₄ and N₂.

[0184]FIG. 7A shows a shape of a hole 9A expected to be formed bysetting the volume flow ratio between the CH₃NH₂ gas and the (CH₄+N₂)gas to CH₃NH₂ <<(CH₄+N₂), FIG. 7B shows a shape of a hole 9B expected tobe formed by setting the volume flow ratio between the CH₃NH₂ gas andthe (CH₄+N₂) gas to CH₃NH₂>> (CH₄+N₂), and FIG. 7C shows a shape of ahole 9C expected to be formed by setting the volume flow ratio betweenthe CH₃NH₂ gas and the (CH₄+N₂) gas to CH₃NH₂=(CH₄+N₂), namely, expectedin Embodiment 4.

[0185] When the CH₃NH₂ gas and the (CH₄+N₂) gas are mixed insubstantially the same volume flow ratio, it is obvious that the hole 9Chas a forward taper cross-section as is shown in FIG. 7C. Also, it isunderstood that the angle of the forward taper cross-section can beadjusted to a desired angle by controlling the mixing ratios of theCH₃NH₂ gas and the (CH₄+N₂) gas.

[0186]FIG. 8 shows a RIE lag characteristic expected to be obtained when100% of the etching gas is the (CH₄+N₂) gas (shown with a broken linemarked with ∘), a RIE lag characteristic expected to be obtained when100% of the etching gas is the CH₃NH₂ gas (shown with a broken linemarked with ▴) and a RIE lag characteristic expected to be obtained whenthe etching gas includes the CH₃NH₂ gas and the (CH₄+N₂) gas in the sameratio (shown with a solid line). In all the cases, the values arestandardized to a hole diameter of 0.24 μm.

[0187] The RIE lag characteristic obtained by using 100% of the CH₃NH₂gas is not very good, but the RIE lag characteristic can be improved bymixing the CH₃NH₂ gas with the (CH₄+N₂) gas. Also, a desired RIE lagcharacteristic can be obtained by controlling the mixing ratios of theCH₃NH₂ gas and the (CH₄+N₂) gas.

[0188] As described so far, since the etching gas including the CH₃NH₂gas and the (CH₄+N₂) gas is used in Embodiment 4, a forward tapercross-section of a hole and a small RIE lag characteristic can be bothrealized. Also, the angle of the forward taper cross-section and the RIElag characteristic can be controlled by adjusting the mixing ratios ofthe CH₃NH₂ gas and the (CH₄+N₂) gas.

[0189] There is a relationship between the angle of the forward tapercross-section and the RIE lag characteristic that one is improved whenthe other is degraded, namely, there is a reciprocal relationshiptherebetween. Specifically, under conditions for increasing the angle ofthe forward taper cross-section, the RIE lag characteristic isincreased, and under conditions for reducing the angle of the forwardtaper cross-section, the RIE lag characteristic is reduced.

[0190] However, an actually required taper cross-section is a verticalcross-section or a slightly forward taper cross-section (an ideal taperangle is generally considered to be 89 degrees). Therefore, when the(CH₄+N₂) gas is mixed with the CH₃NH₂ gas in an amount slightly smallerthan that of the (CH₄+N₂) gas, a taper cross-section derived from theCH₃NH₂ gas and a very small RIE lag characteristic (shown with the solidline in FIG. 8) derived from the (CH₄+N₂) gas can be both realized.

[0191] Although the CH₃NH₂ (methylamine) gas is mixed with the CH₄ gasand the N₂ gas in Embodiment 4, the same characteristic as that obtainedby mixing methylamine can be obtained by mixing one of or a combinationof gases containing, as a principal constituent, a compound includingcarbon, hydrogen and nitrogen, such as dimethylamine ((CH₃)₂NH),trimethylamine ((CH₃)₃N) and triethylamine (C₂H₅NH₂)- Methylamine,dimethylamine, trimethylamine and triethylamine can be very convenientlyused because they can be taken out as a gas at a pressure of 1 atm and atemperature of 25° C. (room temperature). The boiling points ofmethylamine, dimethylamine, trimethylamine and triethylamine are −6.3°C., +7.4° C., +2.9° C. and +16.6° C. respectively.

[0192] Alternatively, the CH₃NH₂ (methylamine) gas may be replaced withany of propylamine (C₃H₇NH₂), a gas of the nitrile family such as C₂H₃N,C₃H₃N and C₃H₅N, a gas of the diamine family such as C₂H₈N, and a gasincluding four or more carbon atoms such as C₄H₅N, C₄H₇N, C₄H₁₁N, C₅H₇Nand C₅H₉N.

[0193] The etching method for any of Embodiments 1 through 4 provides ahigh performance process. A process optimal to the environment ispreferably selected in comprehensive consideration of the requirementfor fine processing (the degree of refinement) in semiconductorfabrication, a fabrication cost (such as an employed gas supply systemand cost of employed gases) and fabrication environments (such asmaintenance of a gas line and use of a system for exhausting a gas).

[0194] Embodiment 5

[0195] A method for etching an organic film according to Embodiment 5 ofthe invention will now be described.

[0196] In the method for etching an organic film of Embodiment 5, amixed gas including, as principal constituents, a CH₄ gas, a N₂ gas anda rare gas (such as an Ar gas) is used as the etching gas, so as to etchan organic film with plasma generated from the mixed gas. Exemplifiedetching conditions in Embodiment 5 are:

[0197] Plasma etching system: NLD plasma etching system

[0198] Type of etching gas and flow rates per minute in standardcondition:

CH₄:N₂:Ar=30 ml:70 ml:200 ml

[0199] Antenna power: 1000 W (13.56 MHz)

[0200] Bias power: 200 W (2 MHz)

[0201] Pressure: 0.4 Pa

[0202] Substrate cooling temperature: 0° C.

[0203] An effect of the etching method for Embodiment 5 will now bedescribed.

[0204] In the etching method for any of Embodiments 1 through 4, a CH₄gas is mixed in the etching gas so as to realize a small RIE lagcharacteristic, and owing to the CH₄ gas, a deposition is easily adheredonto the inner walls of a reaction chamber of the etching system, whichcauses a problem of generation of particles.

[0205] When the etching gas further includes a rare gas as in Embodiment5, the deposition rate of the deposition formed on the inner walls ofthe reaction chamber can be lowered.

[0206] Now, the mechanism for lowering the deposition rate of thedeposition obtained by mixing a rare gas in the etching gas will bedescribed.

[0207] When a rare gas is mixed in the etching gas, partial pressures inthe plasma of the etching gas and atoms or molecules dissociated fromthe etching gas can be lowered in the vicinity of the inner walls of thereaction chamber, and hence, the deposition rate of the deposition islowered. From this point of view, any of He, Ne, Ar, Kr, Xe and Rn maybe used as the rare gas.

[0208] Furthermore, when a rare gas is mixed in the etching gas, themixed rare gas is ionized in the plasma, and the ions of the rare gasare accelerated by a plasma sheath electric field formed in the vicinityof the inner walls of the reaction chamber so as to collide with theinner walls of the reaction chamber. Therefore, the deposition depositedon the inner walls of the reaction chamber is removed through physicalsputtering. From this point of view, any of Ne, Ar, Kr, Xe and Rn may beused as the rare gas. Since He is small in its inertial mass, it cannoteffectively remove the deposition through the physical sputtering.

[0209] <Reason Why Straight Chain Saturated Hydrocarbon is Preferred asMolecule Including Carbon and Hydrogen>

[0210] In each of Embodiments 1 through 5, CH₄ is used as a moleculeincluding carbon and hydrogen to be mixed in the etching gas, but themolecule including carbon and hydrogen is not limited to CH₄ but may bewidely selected from straight chain saturated hydrocarbons.

[0211] Now, the reason why the straight chain saturated hydrocarbon ispreferred as the molecule including carbon and hydrogen will bedescribed.

[0212] In weakly ionized nonequilibrium plasma used in the fineprocessing, dissociation within the plasma is proceeded in proportion tothe number N of collisions between molecules and electrons.Specifically, as the collision number N is larger, the dissociation isfurther proceeded. The collision number N is expressed as a valueobtained by multiplying residence time r of the gas molecules by acollision frequency V (the number of times of collisions per unit time(one second)), namely, N=τ×ν, wherein τ is a value in proportion to thevolume V and the pressure P of a reaction chamber and in inverseproportion to an exhaust amount Q and τ=P·V/Q; and the collisionfrequency v is expressed by a product of the electron density Ne of theplasma and the dissociation rate (σ_(dis)·v_(e)) peculiar to a molecule.Therefore, N=τ×Ne×(σ_(dis)·v_(e)). The dissociation rate (σ_(dis)·v_(e))peculiar to a molecule is determined on the basis of the dissociationcross-sectional area σ_(dis) peculiar to the molecule and convolution ofthe electron speed (energy) distribution v_(e).

[0213] Accordingly, in plasma with the same electron density and thesame electron speed distribution, the proceeding of the dissociation isuniquely determined depending upon the residence time τ of a gas.

[0214] In a straight chain saturated hydrocarbon, when n→∞, H/C=2.Specifically, the H/C ratio in a straight chain saturated hydrocarbonalways has a value larger than 2, namely, H/C>2.

[0215] The effects of this invention obtained because the moleculeincluding carbon and hydrogen satisfies H/C >2 is understood as follows:

[0216] The first effect is the following: The composition ratio betweencarbon and hydrogen in plasma generated from a gas of the moleculeincluding carbon and hydrogen and satisfying H/C>2 also satisfies H/C>2.Accordingly, on a time average basis, carbon and hydrogen are suppliedonto an etch target surface in a ratio of H/C>2. Thus, a state wherecarbon is excessively supplied can be avoided on the etch targetsurface. As a result, a phenomenon of stopping the etching, namely, theetch stop, never occurs.

[0217] The second effect is the following: Molecules generated throughdissociation in plasma are supplied onto an etch target surface in theform of ions or radicals. A molecule C_(x)H_(y) (wherein x and y arepositive integers, 1≦x≦n and 1≦y≦2n+2) generated from a gas of themolecule satisfying H/C>2 has a straight chain single bond alone.

[0218] Now, as the premise of the description of the reason why astraight chain saturated hydrocarbon exhibits the effects in the etchingaccording to any of Embodiments 1 through 5, dissociation of a moleculeproceeded in plasma will be examined.

[0219] In order to practically supply a gas to a plasma etching system,it is necessary to generate a vapor pressure minimally required forenabling a mass flow at 1 atm and room temperature. For this purpose, ina molecule represented by C_(x)H_(y) (wherein x and y are positiveintegers), it is effective that x is equal to or smaller thanapproximately 5.

[0220] Therefore, by exemplifying a gas of a molecule in which thecomposition ratio of C is 5 or less, it will be examined how thedissociation is proceeded and what a dissociation product is generatedas well as how the dissociation product affects the etchingcharacteristics (the RIE lag characteristic and the cross-section of ahole).

[0221] (1) In straight chain saturated hydrocarbons (such as CH₄, C₂H₆,C₄H₁₀ and C₅H₁₂), a C—C bond or a C—H bond is cut through collisionbetween molecules and electrons, and hence, an introduced gas issuccessively decomposed into a molecule having a smaller compositionratio of C. As a result, molecules having a straight chain C—C bond,such as C_(x)H_(y), CH, CH₂, C and H, are generated. In particular, inthe case of molecules of straight chain saturated hydrocarbons where thecomposition ratio x is approximately 5 or less, most of the moleculesare probably decomposed into CH, CH₂, C and H. Such low molecular weightradicals have much lower probability to adhere onto a substance than ahigher molecular weight radicals such as C_(x)H_(y), and hence aresupplied (transported) to an etch target surface on the bottom of arecess with a high aspect ratio. As a result, a deposition film isuniformly formed on the wall and the bottom of the recess. Accordingly,a vertical etch shape can be realized, and the etching rate does notdepend upon the opening diameter or the aspect ratio, namely, etchingwith small RIE lag can be realized.

[0222] (2) In C₂H₂, C₃H₄, C₄H₂, C₄H₄, C₄H₆ and C₅H₈ (hereinafterreferred to as a first group of hydrocarbons for convenience), H/C≦1.5,and thus the H/C ratio is smaller than in the straight chain saturatedhydrocarbon, and hence, a large number of molecules generated throughdissociation in the plasma include a double bond. When introduced gasmolecules include a double bond or a triple bond, the rate ofdecomposing the gas molecules is lowered in accordance with the numberof the double or triple bonds. Therefore, as compared with the straightchain saturated hydrocarbon, the ratio of the amount of generated CH,CH₂, C and H to the amount of generated large molecules represented byC_(x)H_(y) is lowered. Also, the radicals of the molecules including thedouble or triple bonds have larger probability to adsorb onto asubstance, and hence are not supplied (transported) to the bottom of arecess with a high aspect ratio but are mainly deposited on the upperportion of the wall of the recess. Therefore, the effective aspect ratioof the recess is further increased, so that the amount of radicalssupplied to the bottom of the recess can be largely reduced. As aresult, the recess attains an excessively taper cross-section, whichleads to shape defect, or large RIE lag is caused. Therefore, in anextreme case, the phenomenon where the etching is terminated, namely,the etch stop, is caused. Accordingly, none the first group ofhydrocarbons is preferred for realizing the etching with small RIE lag.

[0223] (3) C₃H₆, C₄H₈ and C₅H₁₀ are classified into two types: straightchain hydrocarbon and cyclic hydrocarbon.

[0224] In straight chain C₃H₆, C₄H₈ and C₅H₁₀ (hereinafter referred toas a second group of hydrocarbons for convenience), the H/C ratio is 2,which accords with the H/C ratio of the straight chain saturatedhydrocarbon. However, since each of these molecules includes one doublebond, the ratio of the amount of generated CH, CH₂, C and H to theamount of generated large molecules represented by C_(x)H_(y) is loweredsimilarly to the first group of hydrocarbons, and molecules including adouble bond, such as C₂H₄ and C₂H₂, are generated. Accordingly, thesupply (transportation) of the radicals to the bottom of a recess with ahigh aspect ratio and the formation of a deposition film are similar tothose obtained by the first group of hydrocarbons. In particular, whenC₂H₄ is generated and ions reach from the plasma with a large number ofC₂H₄ adhered onto an etch target surface, C₂H₄ are bonded through ioncollision, so as to easily form a polymer represented by (CH₂)_(n).Therefore, similarly to the first group of hydrocarbons, none of thesecond group of hydrocarbons is preferred for realizing etching withsmall RIE lag.

[0225] On the other hand, cyclic C₃H₆, C₄H₈ and C₅H₁₀ (hereinafterreferred to as a third group of hydrocarbons for convenience) arechemically bonded through a single bond alone to form saturatedhydrocarbons. Also, in the third group of hydrocarbons, the H/C ratio is2. In these molecules, the cyclic C—C bond can be easily cut throughdissociation caused by electron collision, and molecules whose C—C bondis cut can easily form C₂H₄ or CH₂. In particular, C₄H₈ has adissociation path to be decomposed into two C₂H₄ at a time, and hence, alarger amount of C₂H₄ or CH₂ are generated than in the first or secondgroup of hydrocarbons. As described above, since the radicals of C₂H₄not only have high probability to adsorb onto a substance but also areeasily bonded to each other, they can be easily deposited on the upperportion of the wall of a recess without being supplied (transported) tothe bottom of the recess with a high aspect ratio. Accordingly, none ofthe third group of hydrocarbons is preferred for realizing etching withsmall RIE lag.

[0226] (4) As described above, C₂H₄ is a molecule working as afundamental cause for shape defect and a large RIE lag characteristic.Although it depends upon the degree of dissociation of plasma, theconcentration of a supplied gas is high in plasma generally used.Accordingly, C₂H₄ exhibits the most serious effect to deposit on theupper portion of the wall of a recess without being supplied(transported) to the bottom of the recess with a high aspect ratio amongthe various hydrocarbons. Accordingly, C₂H₄ is not preferred not onlybecause a vertical or forward taper cross-section cannot be obtained butalso because a small RIE lag characteristic cannot be attained.

[0227] As described above, when one of or a combination of straightchain saturated hydrocarbons represented by C_(n)H_(2n+1) (wherein n isa positive integer), such as CH₄, C₂H₆, C₄H₁₀ and C₅H₁₂, is used as themolecule including carbon and hydrogen, a recess with a vertical orforward taper cross-section can be formed with a very small RIE lagcharacteristic.

[0228] A gas used in the plasma etching can be optimally selectedbasically depending upon the method or system for exciting the plasma.As a plasma system can more highly excite the plasma, a higher molecularweight gas can be used, and hence, a gas to be used can be selected froma larger range. Specifically, when a plasma etching system capable ofhigh excitement, such as an inductively coupled plasma etching system, asurface wave plasma etching system, an NLD plasma etching system, acapacity coupled parallel plate etching system using RF and an ECRplasma etching system, is used, a gas to be used can be selected inaccordance with the actually used power (energy).

[0229] Furthermore, as the C/H ratio (composition ratio) of the moleculeincluding carbon and hydrogen is larger, the ability to generate adeposition on the bottom of a recess is increased. Accordingly, themolecular weight and the C/H ratio are optimally selected in accordancewith the density of plasma to be used.

[0230] Moreover, an organic film to be etched by the etching method forany of Embodiments 1 through 5 may include atoms such as N, O, B and Sor halogen atoms such as F, Cl and Br as far as the organic film mainlyhas a framework of carbon and hydrogen.

[0231] Also, the effects of this invention are described in each ofEmbodiments 1 through 5 on the basis of the result obtained by using theetching gas in the NLD plasma etching system. However, the method foretching an organic film of any of Embodiments 1 through 5 is applicableto use of any plasma etching system, such as a parallel plate reactiveion etching system, a narrow-gap or two-frequency type parallel platereactive ion etching system, magnetron enhanced reactive ion etchingsystem, an inductively coupled plasma etching system, an antenna coupledplasma etching system, an electron cyclotron resonance plasma etchingsystem and a surface wave plasma etching system.

[0232] Embodiment 6

[0233] A method for fabricating a semiconductor device according toEmbodiment 6 of the invention will now be described with reference toFIGS. 9A through 9C and 10A through 10C.

[0234] First, as is shown in FIG. 9A, a laminated metal interconnectconsisting of a first barrier metal layer 12, a metal film 13 and asecond barrier metal layer 14 is formed on a semiconductor substrate 11,and an organic film 15 is formed on the metal interconnect. Thereafter,a silicon oxide film 16 is formed on the organic film 15 as is shown inFIG. 9B.

[0235] Next, after forming a resist pattern 17 on the silicon oxide film16 by a know lithography technique as is shown in FIG. 9C, the siliconoxide film 16 is subjected to plasma etching (dry etching) by using theresist pattern 17 as a mask, thereby forming a mask pattern 16A from thesilicon oxide film 16 as is shown in FIG. 10A. The type of etching gasto be used in the plasma etching is not herein specified, and a gasincluding fluorocarbon may be used.

[0236] Then, the organic film 15 is subjected to plasma etching usingplasma generated from the etching gas used in the etching method for anyof Embodiments 1 through 5, namely, an etching gas including, as aprincipal constituent, a mixed gas of a molecule including carbon andhydrogen and a molecule including nitrogen, and by using the resistpattern 17 and the mask pattern 16A as masks. In this manner, recesses18 each having a vertical or forward taper cross-section to be used as avia hole or an interconnect groove are formed in the organic film 15 asis shown in FIGS. 10B and 10C. This etching is carried out under etchingconditions of any of Embodiments 1 through 5. Since the resist pattern17 is made from an organic compound, it is removed during the etching ofthe organic film 15.

[0237]FIG. 10B shows a state in the middle of the etching of the organicfilm 15, and under the etching conditions of any of Embodiments 1through 5, the etching can be carried out at a constant etching rateregardless of the diameters of the openings of the recesses 18. A dashedline shown in FIG. 10B corresponds to a reference line for indicating apredetermined depth in the organic film 15.

[0238]FIG. 10C shows a state where the etching of the organic film 15 iscompleted, and under the etching conditions of any of Embodiments 1through 5, the etching is proceeded at substantially the same timing tothe surface of the second barrier metal layer 14 regardless of thediameters of the openings of the recesses 18. Therefore, desiredover-etching can be carried out regardless of the diameters of theopenings of the recesses 18.

[0239] Accordingly, the organic film can be stably processed withoutcausing excessive or insufficient etching and without depending upon thediameters of the openings of the recesses 18, and hence, a large processmargin (process window) can be realized.

[0240] Thereafter, although not shown in the drawings, after cleaningthe surfaces of the recesses 18 and the mask pattern 16A, a thirdbarrier metal layer of TiN or TaN is formed on the walls of the recesses18 by sputtering or CVD. Then, a metal material film is deposited on theentire surface of the mask pattern 16A by the CVD or plating so as tofill the recesses 18, and a portion of the metal material film exposedon the mask pattern 16A is removed by the CMP. Thus, a connection plugor metal interconnect can be formed from the metal material film.Thereafter, when a connection plug and a metal interconnect arealternately formed by a single damascene method, a multi-levelinterconnect structure can be obtained.

[0241] According to Embodiment 6, it is possible to prevent the problemof the excessive or insufficient etching caused in Conventional Example2 because the etching rate of a recess with a small diameter is lowerthan the etching rate of a recess with a large diameter. Therefore, thereliability of a semiconductor device can be improved, and thesemiconductor device can be fabricated with a large process window.

[0242] Although the connection plug or metal interconnect is formed bythe single damascene method in Embodiment 6, the etching method for anyof Embodiments 1 through 5 is naturally applied to a dual damascenemethod for simultaneously forming a connection plug and a metalinterconnect. Also in this case, the reliability of a semiconductordevice can be improved, and the semiconductor device can be fabricatedwith a large process window.

[0243] Furthermore, in Embodiment 6, the metal film 13 may be formedfrom, for example, a W film, an AlCu film, a Cu film, an Ag film, an Aufilm or a Pt film. Alternatively, the metal film 13 may be replaced witha conducting film such as a polysilicon film.

[0244] Also, in Embodiment 6, the materials for the first barrier metallayer 12 and the second barrier metal layer 14 may be selected so as toaccord with the metal film 13, and for example, a laminated filmincluding a Ti film and a TiN film or a Ta film and a TaN film may beused.

[0245] Moreover, an insulating film of a Si₃N₄ film or the like may beused as a barrier layer instead of the second barrier metal layer 14. Inthis case, the Si₃N₄ film is additionally etched after the etching ofthe organic film for forming the recesses.

[0246] Although the mask pattern 16A is formed from a silicon oxide filmin Embodiment 6, a silicon nitride film may be used instead, whereas thesilicon nitride film is preferably formed from a material having asmaller dielectric constant than a silicon oxide film. From this pointof view, a material with a small dielectric constant such as a-SiC:H ispreferably used.

[0247] Furthermore, in the case where the mask pattern 16A is alsoremoved in removing the portion of the metal material film deposited onthe mask pattern 16A exposed outside the recesses 18 by the CMP, themask pattern 16A may be formed from a material with a large dielectricconstant, such as a metal film of titanium or the like, a siliconnitride film, and a metal nitride film of titanium nitride or the like.

[0248] Embodiment 7

[0249] A pattern formation method (top surface imaging process)according to Embodiment 7 of the invention will now be described withreference to FIGS. 11A through 11C, 12A and 12B.

[0250] First, as is shown in FIG. 11A, an organic film 22 is formed on asemiconductor substrate 21, and a silylation target layer 23 is formedon the organic film 22.

[0251] Next, as is shown in FIG. 11B, the silylation target layer 23 isirradiated with exposing light 25 through a photomask 24 for selectivelyallowing the light to pass, thereby selectively forming a decomposedlayer 26 in the silylation target layer 23.

[0252] Then, as is shown in FIG. 11C, with the substrate temperatureincreased, a gaseous silylation reagent 27 is supplied onto the surfaceof the silylation target layer 23, so as to selectively silylate anon-decomposed portion (a portion excluding the decomposed layer 26) ofthe silylation target layer 23. Thus, a silylated layer 28 serving as amask layer is formed.

[0253] Instead of silylating the non-decomposed portion, the decomposedlayer 26 may be silylated to form the silylated layer 28, or thesilylated layer 28 may be directly formed on the organic film 22 withoutforming the silylation target layer 23.

[0254] Next, the organic film 22 is subjected to etching by using plasmagenerated from the etching gas used in the etching method for any ofEmbodiments 1 through 5, namely, the etching gas including, as aprincipal constituent, a mixed gas of a molecule including carbon andhydrogen and a molecule including nitrogen, by using the silylated layer28 as a mask. Thus, openings 29 are formed in the organic film 22 as isshown in FIGS. 12A and 12B.

[0255]FIG. 12A shows a state in the middle of the etching of the organicfilm 22, and since the etching is carried out under the conditions ofany of Embodiments 1 through 5, the etching is conducted in a constantetching rate regardless of the diameters of the openings 29.

[0256]FIG. 12B shows a state where the etching of the organic film 22 iscompleted to form an organic film pattern 22A from the organic film 22.Since the etching is carried out under the conditions of any ofEmbodiments 1 through 5, the openings 29 formed in the organic filmpattern 22A can be prevented from having a larger diameter than openingsof the silylated layer 28 and from having a bowing cross-section.

[0257] Furthermore, since the etching is carried out under theconditions of any of Embodiments 1 through 5, the etching is proceededat substantially the same timing to the surface of the semiconductorsubstrate 21 regardless of the diameters of the openings 29.Accordingly, the organic film pattern 22A can be formed through desiredover-etching, namely, small over-etching, regardless of the diameters ofthe openings 29. In other words, large over-etching, which is requiredin the conventional etching method with a large RIE lag characteristic,can be avoided, so as to reduce a dimensional difference in transferringa pattern. As a result, the fine organic film pattern 22A can be formedhighly precisely.

[0258] Moreover, since the organic film can be stably processed withoutcausing excessive or insufficient etching and without depending upon thediameters of the openings 29, the process can attain a large processmargin (process window).

[0259] Although the semiconductor substrate 21 is formed from a siliconsubstrate in Embodiment 7, a glass substrate used in a liquid crystaldisplay panel or the like or a substrate of a compound semiconductor maybe used instead.

[0260] The method described in Embodiment 7 is the top surface imagingprocess where the mask layer formed on the organic film is the silylatedlayer formed in an exposed or unexposed portion of the organic film.Instead, a three-layer resist process may be employed, so as to use, forexample, a patterned silicon oxide film as the mask layer formed on theorganic film.

What is claimed is:
 1. A method for etching an organic film comprising astep of etching an organic film by using plasma generated from anetching gas containing a first gas including a straight chain saturatedhydrocarbon and a second gas including a nitrogen component.
 2. Themethod for etching an organic film of claim 1 , wherein said etching gasfurther contains a gas including a compound including carbon, nitrogenand hydrogen.
 3. The method for etching an organic film of claim 2 ,wherein said compound is methylamine.
 4. The method for etching anorganic film of claim 1 , wherein said first gas is a methane gas andsaid second gas is a nitrogen gas.
 5. The method for etching an organicfilm of claim 1 , wherein said etching gas further contains a rare gas.6. A method for fabricating a semiconductor device comprising the stepsof: forming an organic film on a semiconductor substrate; forming, onsaid organic film, a mask pattern including an inorganic compound as aprincipal constituent; and forming a recess in said organic film byselectively etching said organic film by using said mask pattern and byusing plasma generated from an etching gas containing a first gasincluding a straight chain saturated hydrocarbon and a second gasincluding a nitrogen component.
 7. The method for fabricating asemiconductor device of claim 6 , wherein said etching gas furthercontains a gas including a compound including carbon, nitrogen andhydrogen.
 8. The method for fabricating a semiconductor device of claim7 , wherein said compound is methylamine.
 9. The method for fabricatinga semiconductor device of claim 6 , wherein said recess includes a viahole and an interconnect groove formed above said via hole and is filledwith a metal material film by a dual damascene method.
 10. The methodfor fabricating a semiconductor device of claim 6 , wherein said firstgas is a methane gas and said second gas is a nitrogen gas.
 11. Themethod for fabricating a semiconductor device of claim 6 , wherein saidetching gas further contains a rare gas.
 12. A pattern formation methodcomprising the steps of: forming an organic film on a substrate;forming, on said organic film, a mask layer including an inorganiccomponent; and forming an organic film pattern from said organic film byselectively etching said organic film by using said mask layer and byusing plasma generated from an etching gas containing a first gasincluding a straight chain saturated hydrocarbon and a second gasincluding a nitrogen component.
 13. The pattern formation method forclaim 12 , wherein said etching gas further contains a gas including acompound including carbon, nitrogen and hydrogen.
 14. The patternformation method for claim 13 , wherein said compound is methylamine.15. The pattern formation method for claim 12 , wherein said first gasis a methane gas and said second gas is a nitrogen gas.
 16. The patternformation method for claim 12 , wherein said etching gas furthercontains a rare gas.
 17. The pattern formation method for claim 12 ,wherein said mask layer is a silylated layer.